1. Field of the Invention
The present invention relates to image forming apparatuses such as a printer, a copying machine, a facsimile machine and a composite machine including functions of a printer, a copying machine and a facsimile machine. Particularly, it relates to an image forming apparatus comprising a fixing device.
2. Description of the Related Art
Conventionally, as the image forming apparatuses described above, the one disclosed in Japanese Unexamined Patent Publication No. 9-114153 is known. This image forming apparatus comprises a fixing section. The fixing section includes a heating member and a pressing member. A recording sheet, such as a pasteboard-like postcard and an A4-sized standard paper sheet, is nipped between the heating member and a pressing member, and then a toner image is heat-fixed on the recording sheet. Between the fixing section and a discharge portion at a downstream of the fixing section, a guiding member is provided. The guiding member guides the recording sheet from the fixing section to the discharge portion.
In the fixing section, the heating member consists of a heat roller and the like. A favorable heat-fixing is realized by adjusting a temperature of the heating member to an appropriate temperature. Too low a temperature of the heating member causes a fixing defect. On the contrary, too high a temperature of the heating member (for example, 195 degrees Celsius) decreases viscosity of toner particles on a recording sheet, which causes a likelihood that the toner particles are transferred to the heating member. Toner particles transferred to the heating member as described above are adhered to other recording sheet after one rotation of the heating member and thereby contaminates the recording sheet. Namely, it produces an offset phenomenon.
A favorable range of a temperature of the heating member is restricted. For example, transference of an image to an A4-sized regular recording sheet requires the restriction of the target temperature of the heating member within a range from 180 to 190 degrees Celsius. In addition, this range is applicable only for a heat-fixing to a recording sheet having a certain thickness. Different thicknesses of recording sheets prefer different temperatures of the heating member. For example, even if the heating temperature is set within the range described above, in the case where a heat-fixing is performed with that temperature to a recording sheet having a large thickness (for example, a cardboard-like postcard), an adherence of toner particles after the heat-fixing is lowered, in other words, an insufficient fixation occurs. This is because a large thickness of a recording sheet suppresses an elevation of the temperature of the recording sheet.
Such disadvantage might be avoided by changing a temperature of the heating member in accordance with a kind of the recording sheet, namely, setting the temperature higher than usual for a heat-fixing on a relatively thick recording sheet. However, such management of the temperature of the heating member is laborious. Further, if a heat-fixing is applied to a thin recording sheet having a large width at a temperature which has not been reached down to a predetermined level from once raised level, a disadvantageous occurrence of an offset may be caused.